Ultrasonic scanner

ABSTRACT

An ultrasonic pulse is directed into a body and electrical representations of pulse reflections from body interfaces along its path are generated. The ultrasonic signal path is scanned through a volume of the body and position signals indicative of the instantaneous path disposition are generated. The reflection signals are selectively gated in accordance with a predetermined function of the path disposition to provide a display selectively representing desired interfaces situated within a selected contoured portion of the volume. By varying the predetermined function, a specific desired interface surface may be displayed. Provisions for developing a three dimensional display of the selected surface are described.

The present invention is directed to an ultrasonic scanning system fordisplaying an interface or surface located within a body, andparticularly to such an ultrasonic scanner for use as a medicalinstrument.

Ultrasonic scanning systems of various types are well known in the priorart. For example, most prior art medical ultrasonic scanning systemsgenerally utilized may be classified as A-type or B-type. In an A-typescanner, a fixed transducer provides an ultrasonic pulse which isdirected along a fixed path into the body. The time-of-return forreflections from internal organic interfaces are detected to provide anindication of the distance to such interfaces. In a B-type scanner, apulsed ultrasonic beam is swept in a single direction, and, as in theA-type scanner, the successive distances (ranges) to reflecting organicinterfaces are determined by standard intervalometer methods. TheseB-type scanners typically provide an indicia of the interface by, ineffect, plotting the detected distances against the position of the beampath. Various B-type scanners have included a real time display and haveeffected scanning electrically, for example, by use of a phasedtransducer array.

In the present invention, a volume is scanned in two dimensions and thethree-dimensional contours of an interface surface are depicted on atwo-dimensional display. Selective range gating, in accordance with apredetermined and/or variable function of the instantaneous scanposition, is utilized to select and contour the volume actually depictedwithin the display. Such selective contouring eliminates "hash" or"clutter" reflections from interfaces other than that of interest. Forexample, in terms of a Cartesian coordinate system, assuming the Z axisto be pointed into the body and the scan performed in the X, Y plane,the gating circuits selectively pass reflections from interfaces locatedwithin a range of Z coordinates determined as a function of the X and Yposition of the signal path.

It is noted that in the human body, most organs are roughly spherical inshape. Accordingly, it has been found more convenient in a medicalenvironment to operate in a polar coordinate system, in effect,raster-scanning a solid angle within the body. The scanning operation isthen described in terms of the angular disposition of the signal pathdefined by the angles α (with respect to the X direction) and θ (withrespect to the Y direction) and the radial distance R from thetransducer. The gating function is performed in the radial direction.

Such selective gating allows for a display showing only the surface of adesired interface. Further, the scanning function can be varied by anoperator to fit the shape of a particular surface.

It has now been recognized by the present inventor that the magnitude(intensity) of the reflection from an interface is a function of theangle of the beam with respect to a tangent plane or normal line to thereflecting surface. When the angle of incidence between the beam and anormal line is small, the itensity of retro-reflection is high.Conversely, when the angle between the beam and a normal line is large,the retro-reflection is of low intensity. Thus, after compensating fornormal attenuation of the ultrasonic signals due to transmission withinthe body, the resultant varying intensity of reflection can be displayedin a raster scan manner to provide an actual molded or varying greylevel "picture" of the surface in question.

An exemplary embodiment of the present invention will hereinafter bedescribed in conjunction with the following drawings, wherein likenumerals denote like elements and:

FIG. 1 is a pictorial schematic of a transducer scanning mechanism andthe solid angle of volume scanned thereby;

FIG. 2 is a schematic block diagram of a scanner system in accordancewith the present invention;

FIG. 3 is a schematic diagram of a suitable gate control circuit;

FIG. 4 is a schematic diagram of another suitable gating control circuitfor effecting contour gating; and

FIGS. 5a, 5b and 5c are exemplary plots versus time of various voltagesV₁, V_(x), and V₃ associated with the circuit of FIG. 4.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

Properly controlled phased arrays of fixed transducers may be used forscanning the ultrasonic beam. On the other hand, a mechanically scannedbeam may also be employed. The particular mechanism employed for thescanning function per se is considered to be conventional, although onepossible arrangement is depicted in the drawings.

Referring now to FIG. 1, a conventional ultrasonic transducer 10 ismounted in a gimbal like mechanism which provides two scanning degreesof freedom. The transducer can thus be scanned through an angle of 2αwith respect to, for example, the X-direction and through an angle of 2θwith respect to, for example, the Y-direction. More specifically,transducer 10 is mounted in an inner ring 14 having shaft-likeprojections 16 and 18. Projections 16 and 18 are rotatably mounted in anouter ring 20, and respectively cooperate with a motor 22 of aconventional type and a sine/cosine generator 24. Sine/cosine generator24 may be of any conventional type, but is preferably of the typecomprising a light and detector system cooperating with a templateaffixed to projection 18. The rotation of projection 18 causes a shapedaperture in the template to progressively move into or out of a lightpassing relationship with the detector to produce a signal indicative ofthe sine or cosine of the rotational angle of projection 18. Outer ring20 has affixed thereto shaft projections 26 and 28, which are rotatablymounted in frame or housing 30. Projections 26 and 28, respectivelycooperate with a conventional motor 32 and sine/cosine generator 34.

Motor 22 provides an oscillatory motion in θ of transducer 10, whilemotor 32 sweeps transducer 10 in α, to provide a raster scan of a solidangle in the body. The scanning mechanism is suitably handheld andmanually positioned against the body, although mechanical positioningapparatus can, of course, be utilized. It should be appreciated that thesweeping action of transducer 10 in α could be manually effected by theoperator rather than by use of motor 32. A water bag or gel 36 may beused to reduce attenuation in air gaps otherwise present between thetransducer and body.

Referring now to FIG. 2, motors 22 and 32 are driven by suitableconventional motor drive circuitry 38, cooperating with suitableconventional timing logic 40. Cosine θ and cosine α signals fromsine/cosine generators 23 and 34 may be utilized as feedback signals, inaccordance with conventional techniques, for motor drive circuitry 38and are also applied to timing logic 40.

Timing logic 40 may be special hardwired logic or a conventionalprogrammed microprocessor or minicomputer. Responsive to the positionsignals, that is, the cosine θ and the cosine α signals, timing logic 40operates to generate trigger pulses to a conventional pulser 42 such asMetrotek MP 203 type pulser. Signals from pulser 42 are applied, througha conventional T-coupler or circulator 44 to transducer 10 to effectgeneration of an ultrasonic pulse. Timing logic 40 generates a firsttrigger pulse to pulser 42 when transducer 10 is in the far upperportion of the scan as determined by the position signals, andthereafter provides trigger signals in accordance with a predeterminedangular displacement within the scan. The time period between pulses ischosen in accordance with the depth of the selected interface within thebody so as to permit reception of corresponding reflected signals priorto generation of the next pulse.

As is well known in the art, the distance to a reflecting interface isdirectly proportional to time required for the roundtrip of a pulsebetween the transducer and the interface. More specifically, the rangedistance R is equal to one-half the roundtrip transit period multipliedby the velocity of sound in the body. However, a portion of theultrasonic pulse is reflected back to the transducer from each interfacealong its path. Reflections from the interfaces other than the interfaceof interest (hash) are thus a potential source of confusion in thedisplay. In order to provide selectivity between the various interfaces,a range gating system is utilized. In general, range gating systems perse are well known in the radar and sonar arts. Reflections from thevarious interfaces are received by transducer 10 and applied throughT-coupler 44 and a variable gain amplifier 46 to a gating device 48 suchas an FET. Gate 48 is selectively rendered conductive for apredetermined second time period after a first time period whichcorresponds to the roundtrip transit time for a selected location on thenear side of the area of interest. The gate is thus opened for a timeperiod corresponding to a range R to R+ΔR of distances from thetransducer. Reflections from interfaces outside of that range areblocked to insure a display of only those interfaces located within theselected range.

Gate device 48 is controlled by a gate control circuit 50. A simpledelay circuit for providing a range gate corresponding to a sphericalsurface section is shown in FIG. 3. This circuit (50a) comprises twoserially connected one-shots 52 and 54, suitably Texas Instruments SN74123 type chips. One-shot 52 is triggered by the timing logic triggerpulse used in generating the ultrasonic pulse. The duration of theoutput signal of one-shot 52 is made to correspond to the roundtripdistance between transducer 10 and the near side of the selected area ofinterest. The negative-going transition at the output of one-shot 52 isutilized to trigger one-shot 54, which produces a pulse having aduration corresponding to the thickness ΔR of the selected volume. Therespective durations of the one-shot output signals may be suitablycontrolled by the operator through potentiometers 56 and 58.

Such a gate control circuit 50a will, during the course of the scan,gate reflection signals corresponding to a portion ΔR of the solid anglesuch as shown in FIG. 1.

Referring again to FIG. 2, the gated output signals are applied to adisplay means 68. Display 68 comprises a scan converter 70 andconventional CRT display 72. Scan converter 70 operates to record thegated data at the scan rate of the ultrasonic system, and then to applysuch data to CRT display 72 at rates compatible with the CRT rasterscan.

In accordance with one aspect of the present invention, the grey levelor beam intensity of the CRT display is controlled in accordance withthe amplitude of the gated signals to provide a molded or threedimensional pictorial depiction of interface surfaces within theselected portion of the scanned volume. The retro-reflection of theultrasonic signals from the reflecting interface are proportional to theangle of incidence between the path of the ultrasonic pulse and a normalto the surface. Where the angle between the beam path and normal issmall, the intensity of retro-reflection is high. When such angle ofincidence is large, the retro-reflection is of low intensity. Thus, bycontrolling beam intensity in the CRT in accordance with the intensityof retro-reflection, an actual grey level "picture" of the interfacesurface is developed. The varying grey level depiction shades theotherwise two dimensional display so as to provide a three-dimensionalvisual effect much like the usual photograph of three-dimensionalarticles.

However, the ultrasonic signal naturally decreases exponentially inintensity as it passes through body tissue due to the usual attenuation.Accordingly, it is desirable to provide compensation for suchattenuation. Such compensation is accomplished in the preferredembodiment by variable gain amplifier 46. Variable gain amplifier 46 maybe a Motorola MC 1350 video IF amplifier which provides a predeterminedgain reduction characteristic. The gain decreases from a maximum byamounts in accordance with a predetermined function beginning at aninput voltage determined by a reference voltage applied to theamplifier. A voltage V_(r), indicative of the desired displacement R ofthe selected volume is utilized as the reference voltage. The gatedsignal or data corresponding thereto is stored in locations in the scanconverter in accordance with the angles θ and α. The scan convertergenerally stores the information in accordance with a Cartesiancoordinate system (X, Y) wherein the X and Y positions for the data aredetermined as follows:

    X=S Tan θ

    Y=S Tan α

The value S is an arbitrary value adjusted such that the display on theCRT screen, from left to right, will be near the edge of the screen whenthe transducer is in its maximum angle θ. For example, if the maximumangle for the transducer is ±20°, then the gain of the scan converter isadjusted such that the maximum value of X is equal to S Tan 20°. Such anadjustment will provide 1 to 1 correspondence between the transmittedultrasonic pulses and the points on the scan converter (and, ultimately,the CRT) raster. It should be appreciated that the roundtrip transittime of the ultrasonic pulses is negligible with respect to themechanical scanning of the transducer 10 and the display rasterscanning.

It should be appreciated that the data can be digitized and stored in ahigh speed mass data storage or memory rather than in a scan converter.The data can then be read out of storage through an appropriate D/Aconverter and displayed on the CRT.

It should also be appreciated, however, that when the interface surfaceto be observed does not readily fit into a spherical section, the simplegate control circuit 40a may not be sufficient to adequately blockunwanted reflections. For example, if spherical section 60 is consideredto be a concave section and the surface of the interface to be observedwere convex, the thickness of spherical section 60 would have to berelatively thick in order to encompass the interface surface.Accordingly, the display could be cluttered with indicia of interfacesother than the interface of interest but within the gated range. It istherefore desirable to contour the range gate in accordance with theparticular surface to be viewed.

Assuming the range gating to begin at a distance R from the transducer,the range gate can be contoured by varying R during the course of thescan. Noting that the distance R at any instance during the scan can beexpressed as a function of the cosines of θ and α, gate control circuit50 can be made, in accordance with one aspect of the present invention,to contour the range gate by activating gate devices at timescorresponding to differing distances R in accordance with the relativedisposition of the path of the ultrasonic pulses within the scannedvolume.

A suitable circuit 50b for generating control signals to gate 48 toeffect a range gate that can be adjusted by the operator to follow aconcave, flat, or convex surface in either the X or Y direction is shownin FIG. 4. The respective cosine signals are applied to respectiveamplifiers A₁ and A₂, together with voltages to adjust the respectiveoutput voltages V₁ and V₂ of amplifiers A₁ and A₂ to provide a zerovoltage at 0°. An example of a typical cosine θ voltage function over±20° scan is shown in FIG. 5(a), along with the corresponding adjustedvoltage V₁. Voltages V₁ and V₂ are respectively applied to one inputterminal of suitable multiplier modules M₁ and M₂, such as Burr Brown BB4205 type multipliers. Multipliers M₁ and M₂ also receive at the otherinput terminals, X contour voltages and Y contour voltages respectively.The X and Y contour voltages generated at levels between a positivemaximum and negative minimum in accordance with potentiometers P₃ andP₄. The output voltages V_(x) and V_(y) of multipliers M₁ and M₂ arerespectively equal to the product of the input signals divided by 10.V_(x) and V_(y), together with voltage V_(r) developed by potentiometerP₅ (indicative of a desired R displacement), are summed by summingamplifier A₃. The resulting voltage V₃ is thus a curve, varying inaccordance with θ and α.

The parameters of the curve are controlled by the operator by adjustingpotentiometers P₃, P₄ and P₅. For example, potentiometers P₃ and P₄ arerespectively connected between positive and negative voltage sources. Byvarying the potentiometer from a center position, the respective contourvoltages can be made positive or negative to provide either concave orconvex curvature. As the magnitude of contour voltage becomes larger,the voltage waveform will become more curved with the cosine of therespective associated angle. Such a contour adjustment of V_(x) isillustrated in FIG. 5b.

The R displacement voltage V_(r) corresponds to the estimated distanceto the interface of interest. As illustrated in FIG. 5c, the Rdisplacement voltage V_(r), in effect, sets the DC level of voltage V₃where θ and α are both zero, i.e., the maximum or minimum of V₃.

The voltage V₃ is applied to an integrator 62 comprising an amplifierA₄, resistor R₁₈ and capacitor C₁. The output of integrator 62, V₄, isgiven by the following equation:

    V.sub.4 =(1/C.sub.1)∫(V.sub.3 /R.sub.18)dt.

However, the controlled integration period is very short such that V₃varies only slightly during the integration and can be considered aconstant. Accordingly, V₄ may be expressed:

    V.sub.4 =[V.sub.3 /(C.sub.1 R.sub.18)]·t

where t is the integration time.

Voltage V₄ is applied to a suitable comparator 64, which also receives avariable reference voltage V₆ generated at potentiometer P₆. The voltageV₆ is set in accordance with the approximate distance of the interfaceto be viewed. Potentiometer P₆ can, if desired, be replaced by a switchto select various approximate starting ranges, such as 2 cm, 4 cm, etc.Comparator 64 generates a transition or pulse when V₄ reaches the levelof V₆, to trigger a one-shot 66, and thereby produce a range gate pulse.The duration of the one-shot output pulse is controlled by apotentiometer P₇.

The integration period is initiated in accordance with the triggerpulses from timing logic 40. The trigger pulses are applied to a flipflop FF1 to set the flip flop and render nonconductive a switchingdevice Q₁ shunted across integrating capacitor C₁. Flip flop FF1 isreset in accordance with the firing of one-shot 66 to end theintegration period, discharge capacitor C₁ and inhibit integrator 62until the next trigger pulse. Thus, the length of time it takesintegrator A₄ to reach threshold level V₆ is generally in accordancewith the equation, A_(x) +B_(y) +C where A_(x) is the x contour voltage,B_(y) is the y contour voltage and C is V_(r).

Thus, circuit 50b effects conduction in gate device 48 for varying timeperiods, corresponding to varying range gates, in accordance with thescanning motion of the transducer.

It should be appreciated that circuit 50b is only one example of manysuitable contour gating control circuits. Suitably programmed digitalcircuits could also be used. In practice, the predetermined functionprovided by the contour gating control circuit is chosen in accordancewith the general expected shape of the objects to be viewed.

It should be noted that while the various conductors showninterconnecting the elements of the drawings are shown in single lines,they are not shown in a limiting sense and may comprise pluralconnections as is understood in the art. Further, it will be understoodthat the above description of one exemplary embodiment of the presentinvention is for illustrative purposes only. The invention is notlimited to the specific form shown, and many modifications may be madein the specific design and arrangement of elements without departingfrom the spirit or scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An ultrasonic scanner system developing atwo-dimensional display of multi-valued signals representative of thevarying intensities of ultrasonic signals reflected from threedimensional interfaces within a body, which intensities vary as afunction of the angle of incidence of incident ultrasonic signals, saidsystem comprising:an ultrasonic transceiver for generating ultrasonicsignals, directing said ultrasonic signals into said body along a pathof predetermined disposition, and generating electrical output signalsindicative of ultrasonic signals reflected back to said transceiver fromsaid reflective interfaces; said transceiver including scanning meansfor scanning said path of predetermined disposition and thus saidultrasonic signals through a predetermined volume within said body;position means generating position signals indicative of the relativeinstantaneous dispositions of said ultrasonic signal path; gating means,responsive to said position signals and to said transceiver outputsignals, for selectively passing only output signals representative ofreflections from interfaces located within a substantial range ofdistances varying within predetermined minimum and maximum distancesfrom said transceiver, which distances at any given time are a functionof the instantaneous relative disposition of said ultrasonic signal paththereby generating gated output signals representative of reflectionsfrom interfaces within a selected volume having a predetermined contour;and display means, responsive to said gated output signals and to saidposition signals providing a two-dimensional display of multi-valuedsignals having more than 2 values and directly representative of andcorrelated to the corresponding varying intensities of ultrasonicsignals reflected from the three-dimensional interface surfaces in saidcontoured volume so as to directly present a shaded two-dimensionaldepiction of said three dimensional interface surfaces.
 2. The system ofclaim 1 wherein said display means includes a cathode ray tuberesponsive to said position signals in deflecting an electron beam ontoa display screen and responsive to said gated signals in modulating theintensity of said electron beam.
 3. The system of claims 1 or 2 furtherincluding compensation means, connected between said transceiver andsaid display means, which adjusts the level of said output signals inaccordance with the approximate distance traveled by the ultrasonicsignals within said body thereby compensating for the attenuation ofsaid ultrasonic signals in said body.
 4. The system of claims 1 or 2,wherein said gating means comprises:a gate device for selectivelypassing said output signals in response to a control signal; firsttiming means generating a first timing signal representing the end of afirst time period occurring after transmission of an ultrasonic signalinto said body, said first time period varying in accordance with apredetermined function of said position signals; and second timing meansfor generating a second timing signal of predetermined duration inresponse to said first timing signal, said second timing signal beingapplied to said gate device as said control signal.
 5. The system ofclaim 4 further comprising range control means for selectively varyingsaid predetermined function whereby said contoured volume can be changedto accommodate a particular desired interface surface.
 6. The system ofclaim 5 further including compensation means, connected between saidtransceiver and said display means, which adjusts the level of saidoutput signals in accordance with the approximate distance traveled bythe ultrasonic signals within said body thereby compensating for theattenuation of said ultrasonic signals in said body.
 7. In an ultrasonicscanning system of the type comprising (a) an ultrasonic transceiver fortransmitting ultrasonic signals into a body along a given path directionand for developing electrical output signals representative ofultrasonic signals reflected back to the transceiver from interfaceslocated along said path, (b) gating means for passing only theelectrical output signals representative of reflections from interfaceswithin a predetermined range of distances along said path, and (c)display means responsive to output signals passed by said gating meansfor providing indicia representative of said interfaces, the improvementwherein:said transceiver includes means adapted for scanning saidultrasonic signals through a predetermined volume and said systemfurther includes position means for generating position signalsindicative of the instantaneous direction of said ultrasonic signalpath; and said gating means includes range control means for varyingsaid predetermined range of distances in accordance with a predeterminedfunction of said position signals, such that the output signals passedthereby are representative of reflections from interfaces within aselected predetermined contoured portion of the scanned volume.
 8. Theimproved ultrasonic scanning system of claim 7 wherein said displaymeans comprises means for generating two-dimensional pictorialrepresentations of three-dimensional interface surfaces contained withinsaid contoured portion of the scanned volume by displaying multi-valuedvisual signals representing the varying intensities of ultrasonicsignals reflected from respectively corresponding portions of saidsurfaces.
 9. The improved ultrasonic scanning system of claims 7 or 8wherein said gating means further comprises means for selectivelyvarying said predetermined function whereby said contoured volumedportion can be varied to accommodate a particular desired interfacesurface in said body.
 10. The improved ultrasonic scanning system ofclaims 1, 2, 7 or 8 further including means for automatically effectingsaid scanning.
 11. In an ultrasonic scanning system of the typecomprising (a) an ultrasonic transducer for transmitting ultrasonicsignals into a body along a given path direction and developingelectrical output signals representative of ultrasonic signals reflectedback to the transducer from interfaces located along said path, (b)gating means for passing only the electrical output signalsrepresentative of reflections from interfaces within a predeterminedrange of distances along said path, and (c) display means responsive tooutput signals passed by said gating means for providing indiciarepresentative of said interfaces, the improvement wherein:saidtransducer includes means adapted to scan said ultrasonic signalsthrough a predetermined volume encompassing a predetermined interfaceand said system further includes position means for providing positionsignals indicative of the instantaneous disposition of said ultrasonicsignal path within the predetermined volume; said gating means includesmeans connected to receive said position signals and to change saidpredetermined range as a function thereof whereby only reflections frominterfaces having corresponding contours are passed through said gatingmeans; and compensation means which compensates for attenuation of saidultrasonic signals such that the amplitude of said gated signals issubstantially independent of the distance to said predeterminedinterface whereby the signal amplitude of output signals passed by saidgating means is indicative of the relative angle between the reflectingpredetermined interface and said ultrasonic signal path; and saiddisplay means includes means for generating multi-valued display signalsrespectively corresponding to the varying amplitude of output signalspassed by said gating means, and means responsive to said positionsignals for visually displaying said multi-valued display signals on atwo-dimensional display at locations thereon respectively correspondingto the disposition of the ultrasonic signal path which caused suchdisplay signal to occur.
 12. The improved ultrasonic scanning system ofclaim 11 wherein said display means generates shaded two-dimensionalpictorial-like representations of the three-dimensional interfacesurfaces within the selected contoured volumed portion.
 13. The systemof claims 11 or 12 wherein said display means comprises a cathode raytube with its electron beam deflected in response to said positionsignals and the intensity of its electron beam varied in response to theamplitude of output signals passed by said gating means.
 14. In thesystem of claims 11 or 12, the further improvement wherein:said gatingmeans includes range means for varying said predetermined range ofdistances in accordance with a predetermined function of said positionsignals such that output signals passed by the gating means representreflections from interfaces within a variably selected portion of saidscanned volume.
 15. The system of claim 14 wherein said display meanscomprises a cathode ray tube with its electron beam deflected inresponse to said position signals and the intensity of its electron beamvaried in response to the amplitude of output signals passed by saidgating means.
 16. The system of claim 14 wherein said gating meansfurther comprises means for selectively varying said predeterminedfunction whereby the contour of the selected volume can be varied toaccommodate a particular desired interface surface.
 17. The system ofclaims 11 or 12 further including means for automatically effecting saidscanning.
 18. Apparatus for developing a visual representation of apredetermined internal surface in a body, said apparatuscomprising:means for transmitting an ultrasonic signal into said bodyalong a predetermined path and for receiving ultrasonic signalsretro-reflected back from surfaces encountered in said body along saidpath; said means for transmitting being adapted for scanning said paththroughout a predetermined volume within said body and encompassing saidpredetermined surface; gating means responsive to a control signal forselectively passing signals derived from said retro-reflected ultrasonicsignals to produce a gated signal representative of reflections fromsurfaces encountered within a predetermined range of distances located aspecific distance from said means for transmitting and receiving; meansfor generating position signals indicative of the instantaneousdisposition of the ultrasonic signal path within said volume; means forgenerating said control signal in accordance with a predeterminedfunction of the position signals such that said gated signal representsreflections from said surfaces within a contoured portion of the scannedvolume, said contoured portion being approximately fitted to theexpected shape of said particular internal surface; and display meansresponsive to said gated signal for providing a visual representation ofsaid predetermined internal surfaces.
 19. The apparatus of claim 18further including:means for varying said predetermined function wherebysaid contoured portion can be approximately matched to a particularsurface.
 20. The apparatus of claims 18 or 19 wherein said display meanscomprises means compensating for attenuation of said ultrasonic signalsin accordance with the approximate distance traveled by the ultrasonicsignals;means generating multi-valued display signals indicative of thevarying amplitude of the compensated and gated signals representingretro-reflected ultrasonic signals; and means relating the multi-valueddisplay signals to the position signals and producing a visual displaydepicting the three-dimensional internal surface being scanned.
 21. Theapparatus of claim 20 wherein said display means comprises a cathode raytube connected to deflect an electron beam in response to said positionsignals and to vary the intensity of such electron beam in response tosaid multi-valued display signals.
 22. A method providing a visualrepresentation of a selected internal surface in a body comprising thesteps of:directing an ultrasonic signal into said body along apredetermined path and generating electrical reflection signalsindicative of ultrasonic signals retro-reflected back from internalsurfaces encountered along said path; scanning said path through apredetermined volume of said body encompassing said selected internalsurface; generating position signals indicative of the instantaneousdisposition of said ultrasonic signal path within said volume;selectively passing said reflection signals through a gate to generate agated signal representing retro-reflections from internal surfaceswithin a predetermined range of distances; varying said predeterminedrange of distances in accordance with a predetermined function of theposition signals such that said gated signal representsretro-reflections from selected internal surfaces within a selectedcontoured portion of said scanned volume; and generating and displayingvisual indicia of said selected internal surfaces within said selectedcontoured portion from said gated signal.
 23. A method as in claim 22further comprising the step of compensating for the approximate distancetraveled by the ultrasonic signals so that the selectively passedreflection signals are substantially independent of such distance.
 24. Amethod as in claims 22 or 23 wherein said visual indicia are potentiallymulti-valued for any given location on a two-dimensinal visual displayand where the value of such indicia for any given location is a functionof the amplitude of retro-reflected ultrasonic signals emanating from arespectively corresponding location on the selected internal surface.25. A method as in claim 24 wherein said predetermined function isselectively varied so as to change the contoured volume to accommodate aparticular desired interface surface.